Sulphuric acid regeneration



SPf 3, 1946. N. TITLESTAD i 2,406,930

\ SULFURIC ACID REGENERATION Filed June 12, 1942 5 Sheets-Sheet 2 l/llllllllix); M Y /26 BY Zn AT oRNEY I sept- 3 1946 N. TrrLEs'rAD 2,406,930

SULFURIC ACID REGENERATION Filed June 12, 19,42 l 5 Shoots-Sheet 3 INVENTOR ATT RNEY Sept. 3, 1946.

N. 'rlTLEsTAn SULFURIG ACID REGENERATION Filed June 12', 1942 5 Sheets-Sheet 4 14,.. WMU AT ORNEY Sept. 3, 1946.- N. TITLEsTAD SULFURIC ACID REGENERATION 5 Sheets-Sheet 5 Filed June 12, 1942 INVENTOR A ORNE( Patented Sept. 3, 1946 giocoso sULPHUaIc Aom aEdENERA'rIoN Nicolay 'ranma white Plains, N. Y., assigner-.to

Chemical Construction Corporation, New York, N. Y., a corporation of Delaware Application June 12, 1942, Serial No. 446,719

(Cl. 23--1'i2) .6 Claims.

This invention relates to a. regeneration meth- `od for the production of sulphuric acid monohydrate or oleum from waste or by-product acid of 80-96% H2S04 content or less. The process is especially Well adapted for the production of oleum suitable for use in the nitration of toluol, phenols and the like for the manufacture of explosives, using as a principal raw material the denitrated sulphuric acid obtained as a by-prod- Y uct therefrom. It should be understood, however, that partially diluted and contaminated sulphuric acid from other sources may be vregenerated into sulphuric acid monohydrate or' oleum by the process of the invention, typical examples being spent alkylation acid from the production of high octane gasoline by condensing isobutane and butylene in strong sulphuric acid spent acid from the absortion of oleniines in the manufacture of isopropyl and isobutyl alcohols and the like.

It is well known to recqncentrate sulphuric acid obtained from the above and similar sources by ordinary concentrating methods, such as by heating in acid proof vessels, by the direct action of hot gases, or by combinations of these two methods. However, such concentration methods can only be used to obtain 93-96% sulphuric acid at the most since it is not feasible to produce stronger acid by ordinary methods of concentration. 'I'his reconcentrated acid must be fortified by S03 from other sources in order to produce 98% sulphuric acid or oleum.

The reconcentration of spent sulphuric acid from nitrating mixtures by ordinary methods followed by fortification with sulphur trioxide always results in the production of a large amount of excess sulphuric acid of 80-96% into sulphur dioxide, oxygen andtwater vapor H2804 content, which acid isv usually dark or discolored, of little or no commercial value, and hardto dispose of. Itis a principal object of the present invention to provide a method of' regenerating strong sulphuric acid of 80-96% HzSOi content into monohydrate or oleum of any desiredstrength by a process which completely avoids the production of such waste acid.

A further important object is the provision ofva regeneration process wherein the water balance can be controlled at any desired value, thereby enabling the operator to produce any desired proportion of oleum of varying strengths, together with sulphuric acid monohydrate if desired, independently of atmospheric conditions prevailing in the plant.

y The underlying principle of the present invention is the decomposition of sulphuric acid 1 tially free from S03 it is necessary to introduce by the reaction.

As distinguished from these reduction proc-a esses, the present invention is directed to the decomposition of strong sulphuric acid of --96% H2504 content by heating the acid in finely divided i'orm to extremely high temperatures. whereby the H2504 is decomposed rapidly to sulphur dioxide, oxygen and water vapor without the necessity of employing reducing agents. I have discovered that sulphuric acid of this strength can be completely decomposed to a mixture of sulphur dioxide, oxygen and water vapor having a high content of sulphur dioxide and oxygen by direct contact of the strong sulphurc acid in finely divided form with the flame of a burning mixture of a combustible hydrocarbon and air preheated to at least 800 F.

The use of preheated air is a very important feature of my invention, for the added heat supplied in this manner augments the normal flame temperature so that the finely divided acid is decomposed at temperatures of 1600-2400 F. or higher. The acid is then rapidly and completely decomposed into a gas mixture containing practically no S03 or undecomposed sulphuric acid but containing sulphur dioxide and oxygen in the proper ratio for treatment in a contact sulphuric acid plant. Moreover, the preheat supplied to the air reduces the quantity 0f fuel that would otherwise be necessary for complete decomposition, and dilution of the furnace gases with carbon dioxide is correspondingly reduced. Thus, for example, by burning sulphuric acid of -93% H2SO4 content in this manner with air preheated to Q-1700 F., a gas mixture is obtained which contains 8-8.5% S02 and 11-11.5% oxygen by volume. p

In order to heat all the sulphuric acid rapidly to the extremely high decomposing temperatures that are necessary to obtain a S02 gas substanthe sulphuric acid in the form of alvapor or a yfinely divided spray which win result in immediate vaporization of the acid upon contact with the flame of the burning hydrocarbon. In one embodiment of this feature Iof my invention I inject a stream of the finely divided strong sulphuric acid into a cone of flame formed by a burning mixture of combustible hydrocarbon and preheated air. 'I'his procedure insures complete vapor-ization of the acid before it can cantact with the refractory lining of the furnace, and thus prevents deterioration of the furnace wall.

Anot en specific embodiment of my invention includes a preliminary vaporization of the strong sulphuric acid by preheated air.' preferably by injecting the acid in finely divided form into the air. stream, after whichthe resulting sulphuric acid vapors are commingled with the flame of the burning hydrocarbon. This preliminary vaporlzation produces a molecular dispersion of the sulphuric acid and facilitates complete decomposition thereof in the furnace. The preliminary vaporization also assists in maintaining a high flame temperature since the heat necessary to supply the latent heat of vaporization of the acid is not suddenly abstracted from the burning hydrocarbon, but is supplied by the preheated air.

Another method that can be employed to insure a high flame temperature is to introduce a hydrocarbon fuel in admixture with the sulphuric acid. This occurs naturally in the treatment of alkylation acid and denitrated acid from the nitration of aromatic hydrocarbons and phenols wherein the concentrated acid contains substantial quantities of hydrocarbons or other combustible material, and the combustion of this material in the furnace permits a corresponding reduction in the amount of additional fuel that must be supplied.

Upon completion of the sulphuric acid decomposition the resulting gas mixture, which contains sulphur dioxide, oxygen, water vapor and carbon dioxide, is preferably cooled by contacting it with a spray of cooling water, with or without preliminary cooling. The water spray serves to condense excess water from the gases, which may then be passed through an electrostatic precipitator of the Cottrell type or through coke boxes to remove any acid mist that may be present. The gases are then further dried by contact with strong sulphuric acid after which they are preheated .to conversion temperatures of 40G-500 C. and passed through a catalytic converter containing a sulphuric acid contact mass of the platinum or vanadium oxide type in order to convert their SO2 content to SQa.

The gases from the converter, in which substantially all the sulphur dioxide has been oxidized to sulphur trioxide, are preferably cooled to temperatures of about 20o-250 C. or'lower in a sulphur trioxide cooler and are then absorbed in strong sulphuric acid in the usual manner to form sulphuric acid monohydrate or oleum. For this purpose two or more absorbers operating in series may be used wherein the rate of feed of the absorbing acid may be adjusted to produce oleum of different strengths. In such cases the gases leaving the last oleum absorber are preferably passed through a nal absorbing tower where part of the spent acid which has been concentrated by ordinary methods to 93-96% H2804 content is added.

The process of my invention will be further described with reference to the accompanyingdrawings, which illustrate certain preferred modifications thereof. In these drawings:

Fig. 1 is a flow diagram wherein the most essential steps of the complete process of oleum production are diagrammatically illustrated;

Fig. 2 is an enlarged diagrammatic illustragiuon Jof the upper part of the furnace shown in Fig. 3 is a diagrammatic illustration of a modified form of furnace wherein the air is preheated by heat exchange with the furnace gases:

Fig. 4 illustrates diagrammatically a further modification wherein the sulphuric acid is vaporizedI into a stream of preheated air before being admitted into the furnace proper;

Fig. 5 illustrates an embodiment wherein the incoming air is preheated by passage through the furnace walls;

Fig. 6 is a modification showing another method of preheating the air while protecting the furnace walls from damage by the high furnace temperature;

Fig. 'I is a modification of the embodiment of Fig. 5 showing a method of injecting a vaporizable liquid hydrocarbon into the furnace by the action of steam; and

Fig. 8 is a. flow sheet showing a method of applying the invention in a nitrating plant.

Referring tn Fig. l, strong sulphuric acid of 8096% H2804 content is subjected to thermal decomposition in a furnace I in accordance with the principles of my invention as outlined above. The acid is preferably injected in the form o f a finely divided spray through' an acid inlet pipe 2, and this spray is surrounded by a cone of flame formed by burning in the burner 3 a combustible hydrocarbon gas introduced through the pipe 4. Flame temperatures of 1600-'2400 F. are insured by the introduction of preheated air through the air inlet 5, the air preferably being preheated to a temperature of within 200-800 F. of the combustion' temperature necessary to obtain complete decomposition of sulphuric acid of the strength being used. A minimum preheating temperature of 800 F. is necessary, and a preferred temperature, range of 1400-1800 F. is usually employed for this purpose.

'Ihe gases from the furnace I may be given a preliminary cooling by heat exchange with the incoming air, as shown in greater detail in Figs. 3 and 4, or a part of their heat content may be recovered by other means. The furnace 'gases are then cooled to relatively low temperatures on the order of 'l0-90 F. or lower by direct contact with a spray of cooling water in the cooling tower IIL-which results in condensing the greater partof their moisture content. 'I'he cooled and partially dehydrated gases are then preferably subjected to the action of an electrostatic field in a precipitator Il for the removal of acid mist.

- after which the remainder of their moisture is removed by contacting them with strong sulphuric acid in the drying tower I2.

The resulting dried gas mixture, containing carbon dioxide and sulphur dioxide and oxygen in proportions suitable for catalytic conversion, is preheated and converted into sulphur trioxide gases in a catalytic converter I3 of any suitable design. The resulting converted gases are the passed through a sulphur trioxide cooler I4 and are introduced successively into the absorption towers I5 and I6 where the S03 is taken up in strong sulphuric acid for the production of oleum. The gases are then passed through a ilnal absorbing tower Il which may be fed with reconcentrated spent sulphuric acid* of Sii-96% 0r with acid from the drying tower I2, wherein the remainder of their sulphur trioxide content may be removed.

As has been stated, it is necessary to obtain a rapid and complete decomposition of the sul.

phuric acid in the furnace I in order 4to prevent excessive loss of sulphur values as sulphur trioxide or distilled sulphuric vacid in the cooling tower. I0 and precipitator Il. One of the most important features of my invention resides in the commingling of a strong sulphuric acid of Sil-96% II2SO4 content in finely divided form with the flame of a burning mixture of a combustible hydrocarbon and preheated air, for I have found that this treatment results in an extremely rapid and complete decomposition of the HzSO4 to sulphur dioxide, oxygen' and water vapor. In the arrangement shown in Fig. 2 this is accomplished by spraying the strong sulphuric acid from the pipe 2 through an atomizing nozzle G, this nozzle being mounted just below the center of the ring type burner 3 wherein a hydrocarbon gas is burned with the aid of preheated air entering through the air conduit 5. This air is supplied through vanes 1 and mixes with the hydrocarbon gas from the pipe 4 to form a cone of flame within the furnace. The acidA spray 9 from the spray nozzle 6 commingles with the llame 8 and is almost instantly vaporized and decomposed under the action of the high flame temperature resulting from the use of preheated air. I

In the modification of Fig. 3 the acid .injecting nozzle and gas burner are identical with/those shown in Fig. 2, but a particularly advantageous method of preheating the incoming air is illustrated. In this modification of theinvention an air preheater 20 is provided in the form of a tubular heat exchanger 2| provided with heat exchange tubes 22 mounted across the conduit 23 1 for the incoming air. Acid gases from the fur' In the modification of Fig. 6 opposed jets of nely divided strong sulphuric acid and hydrocarbon oil are injected from opposite sldes'of a furnace 40 through pipes 4I and 42 respectively. Preheated air at a. temperature sufliciently high to maintain the necessary high decomposition temperatures of 16002400 F. is introduced from passages 43 and 44 in the walls of the furnace through perforations 45 and 46 in the refractory brink lining 41 thereof. By introducing the air in this manner the furnace lining 41 is protected against destruction by .possible localized high furnace temperatures, while the air during its flow through the passages 43 and 44 may be given the necessary degree of preheat to insure a suit- I ably high flame temperature in the furnace.

When a liquid hydrocarbon is used as a fuel Fig. 7 may be employed, and it should be underfurnace I, wherein it is further preheated before admixture with gasfrom the'burner 3. Fig. 4 illustrates a further modification of the sulphuric acid decomposition process of my inl vention wherein a spearate chamber is provided and the resulting preheated air having a. tem

perature of at least 800- F. and preferably 1400- 1800" F. is then admitted to a vaporizing chamber 3| where it is contacted with a spray of strong sulphuric acid of 8096% H2804 content admitted through acid inlet pipe 32. The acid is rapidly vaporized into the air by rea-son of the high temperature thereof, which is well above the boiling point of the acid, and the resulting vapors are admitted through inlet pipe 33 to the furnace where they are commingled with a flame of burning hydrocarbon which is admitted through inlet pipe 34. In practicing this modification of the invention the temperature of the sulphuric acid-preheated air mixture entering the furnace through the pipe 33 is usually lower than in the stood that an injector of this type may befsubstituted in the furnaces shown in Figs. 2, 3 and 5 if desired. Referring to Fig. 7 of the drawings the injector, which is designated generally byreference numeral 50, comprises three concentric pipes: an inner pipe 5 I, an intermediate pipe 52 and an outer pipe 53 adapted to conduct `strong sulphuric acid, fuel oil and steam respectively. A

spiral 54 near the outlet of pipe 52 is adapted to impart a swirling motion to the fuel oil as it is ejected therefrom, and a blast of steam from the outer pipe 53 immediately atomizes the rotating stream of oil and injects it into the furnace I in the form of a fine spray. Preheated air enter-V ing through the passages 55 and 56 is mixed with this oil spray by the action of directing vanes 51, and the ignited oil spray -forms a cone of flame with whichfinely divided acid from the spray nozzle 58 is mixed. Y

From the foregoing it is evident that lthe sulphuric acid decomposition process of my invention is essentially one wherein strong sulphuric acid of about Btl-96% HzSO4 content is heated to temperatures of at least 1600 F., and preferably at temperatures within the range of 16002400 F. by commingling` it in finely divided form with the flame of a burning mixture of a combustible hydrocarbon and air preheated to at least 800 F., the temperature at which the air is preheated preferably being within ZOU-800 'I'he heat-l F. of the desired flame temperature. ing is continued at these temperatures for" a time sufiicent to complete the reaction4 i been decomposed by this process the water can 'formation of sulphuric acid monohydrate nor As a specific example illustrating the practice of my invention, 106 tons of H2804 (as 93% sulphuric acid) are sprayed during a 24-hour day into a combustion furnace provided with suiilcient fuel oil (3700 gallons of fuel oil of .902 sp. gr.) and sumcient air (3400 ou. ft. min. a* standard conditions) to supply the required heat to completely decompose the sulphuric acid. I have found that this reaction goes substantially to completion at temperatures within the range of l600-'2400 F. provided that more than 6%' of oxygen on the dry basis is present in the reaction products. Under these conditions if the f sulphuric acid entering the furnace has a temperature of 150 F. and the entering air is preheated to 1750 F. the flame temperature within the furnace will be about 2200 F. and there will be obtained 5200 cu. ft. per min. (measured urider standard conditions) of gases leaving the furnace with the following composition These gases are cooled to approximately 75 F. by direct contact with cooling water at 70 F. in the cooling tower I0. 'Ihe resulting gases, after passing through the electrostatic precipitator II, drying tower I2, catalytic converter I3 and SO: cooler I4 are absorbed in strong sulphuric acid in the towers I5, I6 and I1.

A complete process for the regeneration of spent denitrated sulphuric acid in conjunction with the operation of a nitration plant is illustrated in Fig. 8 of the drawings, which is a ow sheet illustrating diagrammatically the various steps of the process. In this plant the concentrators are fed with spent denitrated acid oi' about 70% HzSO4 content and supply '790 tons of real H2SO4 per 24 hour day as 93% sulphuric acid. 495 tons of this 93% acid (corresponding to 460 tons H2804) are sprayed into the furnace |0I together with sufficient sulphur or sulphurbearing material to produce 130 tons of H2304 as make-up acid. Thus, for example, 42.5 tons of sulphur may be burned in the furnace |0I per 24 hour day. Alternatively, the sulphurmay be burned in a separate sulphur burner and the heat may be recovered in the heat exchanger |02 wherein the hot gases from the furnace |0| and from the sulphur burner may be mixed and passed in heat exchange relationlwith the incoming air in order to preheat the air to the requisite degree. The resulting sulphur dioxide gas mixture is then cooled to precipitate excessiye moisture in the gas cooler |03 and passed through the mist precipitator |04 and drying towers |05 and |00, after which it is reheated in a heat exchanger |01 and passed through a catalytic converter |00. 'I'he resulting sulphur trioxide gases are cooled in a S03 cooler |09 and passed through absorbers I|0, and ||2 for the recovery of their SO2 content as sulphuric acid monohydrate and oleum.

@Approximately 355 tons of 93% sulphuric acid from the concentrators |00, corresponding to 330 tons of sulphuric acid, are utilized as feed acid for the 98% H2804 absorbing tower, and sulphuric acid monohydrate froml this tower is passed in series through towers III and H0 in countercurrent ilow to the ilow of the S0.: gases. The final product from the plant is thus obtained as 109% sulphuric acid (oleum) in a quantity sumcient to make up for all mechanical losses and entrainment irr the nitration plant, the denitrating plant and the concentrators |00, and thus the entire sulphuric acid requirements of the nitration plant are supplied.

What I claim is:

1. A method of regenerating relatively concentrated sulphuric acid which'comprises in- Jecting said acid in finely divided condition into a combustion zone while simultaneously introducing air and -fuel into said zone in amounts such as to maintain oxidizing conditions and a temperature of at least ,1600 F. therein and free oxygen in the products of combustion, but less than quantities which would dilute the products of combustion to an SO2 content at which the catalytic air oxidation of SO2 to SO: is not self-sustaining, heating the acid in said zone until its thermal decomposition is substantially complete, catalytically oxidizing the sulphur dioxide in the resulting gases by the contact sulphuricacid process, and absorbing the resulting sulphurl trioxide in concentrated sulphuric acid.

2. A method of regenerating relatively concentrated sulphuric acid which comprises injecting said acid in finely divided condition into a combustion zone while simultaneously introducing air and a fuel selected from the group consisting of hydrocarbon and sulphur fuels in amounts such as to maintain oxidizing conditions and a temperature of at least 1600 F. therein and free oxygen in the products of combustion.

but less than quantities which would dilute the products of combustion to an SQ: content at which the catalytic air oxidation of SO: to S03 is not self-sustaining, heating the acid in said combustion zone until its thermal decomposition is substantially complete, catalytically oxidizing the sulphur dioxide in the resulting gases by the contact sulphuricacid process,.and absorbing the resulting sulphur trioxide in concentrated sulphuric acid.

3. A method of regenerating relatively concentrated sulphuric acid which comprises injecting said acid in finely divided condition into a combustion zone while simultaneously introducing air and fuel into said zone in amounts such as to maintain oxidizing conditions and a temperature of at least 1600 F. therein and free oxygen in the products of combustion, but less than quantities which would dilute the products of combustion to an SO2 content at which the catalytic air oxidation of SO2 to SO: is not selfsustaining, heating the acid in said zone until its thermal decomposition is substantially complete, preheating at least a part of said air on its way to said combustion zone by passing it in heat exchange relation with the combustion gases leaving said zone, catalytically oxidizing the sulphur dioxide in the combustion gases by the contact sulphuric acid process, and absorbing the resulting sulphur trioxide in concentrated sulphuric acid.

4. A process according to claim 3 in which the air is preheated to at least 800 F.

5. A regeneration method for weak sulphuric acid which comprises concentrating said weak acid to concentrated acid of 96% HzSOi content, subjecting a part of the concentrated acid to thermal decomposition by injecting it in finely divided condition into a combustion zone while simultaneouslyV introducing air and fuel into said zone in amounts such as to maintain oxidizing conditions and a temperature of at least 1600 F. therein and free oxygen in the products of combustion, but less than quantities which would dilute the products of combustion to an lSO2 content at which the catalytic air oxidation of SO2 to S03 is not self-sustaining, heating the acid in said zone until its thermal decomposition is substantially comp1ete,'catalyti cally oxidizing the sulphur dioxide inthe resulting gases by the contactgsulphuric acid process, and absorbing the resulting sulphur trioxide-in the remainder of theconcentrated acid.

6. A method of regenerating relatively concentrated suiphuric acid which comprises injecting said acid iny neiy divided condition into a combustion zone while simultaneously introducing air and fuel into said zone in amounts sufcient to maintain oxidizing conditions and a temperature of at least 1600 F., to obtain a gaseous mixture containing SO2 and oxygen in amounts suitable for catalytic oxidation to form SO: without further addition of SO2, catalytically oxidizing the SO: to` S03 without such further addition, and recovering S03 as concentrated sulphuric acid.

- NICOLAY TITLESTAD. 

